In recent years, a portable electronic device such as a digital camera, a smart phone, or a tablet device has become remarkably popular. Along with this, demand for an electric storage device such as a secondary battery that can be repeatedly used by charging and is used as a power source for these devices is largely increasing, and demands for higher capacity and higher energy density of the electric storage device are further increasing.
In these electric storage devices, in general, a solution in which an ionic conductive salt is dissolved in an aprotic organic solvent is used as an electrolyte solution.
By the way, in an electric storage device aiming for high-voltage operation, a withstand voltage of an electrolyte solution is a factor determining an upper limit potential of the device. Under a high voltage, an organic solvent and an ionic conductive salt constituting the electrolyte solution are exposed to a high voltage and may be electrically decomposed on a surface of an electrode. An electrolyte solution having a better withstand voltage property is required.
As a technique for improving the withstand voltage of an electrolyte solution, a method for adding various additives to the electrolyte solution has been reported (for example, refer to Patent Documents 1 to 4). However, there is room for further improvement from viewpoints of au effect of improving a withstand voltage and suppression of an increase in internal resistance.
It is known that an ionic liquid has a high withstand voltage. However, the ionic liquid has a problem in terms of ionic conductivity in a range of a low temperature to a normal temperature as compared with a general organic solvent, and is unsuitable for a device operating at a low temperature and a high voltage.